internal/buildcfg: extract logic specific to cmd/go

cmd/go/internal/cfg duplicates many of the fields of
internal/buildcfg, but initializes them from a Go environment file in
addition to the usual process environment.

internal/buildcfg doesn't (and shouldn't) know or care about that
environment file, but prior to this CL it exposed hooks for
cmd/go/internal/cfg to write data back to internal/buildcfg to
incorporate information from the file. It also produced quirky
GOEXPERIMENT strings when a non-trivial default was overridden,
seemingly so that 'go env' would produce those same quirky strings in
edge-cases where they are needed.

This change reverses that information flow: internal/buildcfg now
exports a structured type with methods — instead of top-level
functions communicating through global state — so that cmd/go can
utilize its marshaling and unmarshaling functionality without also
needing to write results back into buildcfg package state.

The quirks specific to 'go env' have been eliminated by distinguishing
between the raw GOEXPERIMENT value set by the user (which is what we
should report from 'go env') and the cleaned, canonical equivalent
(which is what we should use in the build cache key).

For #51461.

Change-Id: I4ef5b7c58b1fb3468497649a6d2fb6c19aa06c70
Reviewed-on: https://go-review.googlesource.com/c/go/+/393574
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/asm/internal/lex/input.go

@@ -50,7 +50,7 @@ func predefine(defines flags.MultiFlag) map[string]*Macro {
 	// Set macros for GOEXPERIMENTs so we can easily switch
 	// runtime assembly code based on them.
 	if *flags.CompilingRuntime {
-		for _, exp := range buildcfg.EnabledExperiments() {
+		for _, exp := range buildcfg.Experiment.Enabled() {
 			// Define macro.
 			name := "GOEXPERIMENT_" + exp
 			macros[name] = &Macro{